Positive electrode for lithium ion secondary battery and lithium ion secondary battery including same

ABSTRACT

Disclosed is a rechargeable lithium ion battery including a positive electrode including a positive active material and a negative electrode including a negative active material. The positive active material includes a lithium-manganese-based compound core and a heat resistant polymer disposed on the lithium-manganese-based compound core. The heat resistance polymer has a glass transition temperature (T g ) ranging from about 80 to about 400° C. The positive active material includes a lithium-manganese-based compound core and an inorganic metal compound as well as the heat resistant polymer with a glass transition temperature (T g ) ranging from about 80 to about 400° C. disposed on the lithium-manganese-based compound core.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Japanese Patent Application No 2008-305718 filed in the Japanese Property Office on Nov. 28, 2008 and Korean Patent Application No. 2009-0112855 filed in the Korean Intellectual Property Office on Nov. 20, 2009, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Field

This disclosure relates to a positive electrode for a rechargeable lithium ion battery and a rechargeable lithium ion battery including the same.

2. Description of the Related Art

Recently, as a rechargeable lithium ion battery that can be used for a mobile phone, a laptop computer, or the like has been required of much higher capacity, there has been an attempt to accomplish the high-capacity and high energy density by setting a high charge, resultantly improving utilization ratio of a positive electrode.

For example, when a lithium manganese-based oxide such as LiMn₂O₄ and the like is used as a positive active material, it may be deposited into a manganese metal by the inserting reaction of lithium ions accompanied with the charge and discharge. As aforementioned, when the manganese metal is deposited and accumulated in a negative electrode, a rechargeable lithium ion battery may have deteriorated cycle characteristics due to destruction of a separator or degradation of the negative electrode.

Like this, when a metal is eluted from a positive active material, the metal may be deposited on a negative electrode or oxidize a separator, resultantly degrading a rechargeable lithium ion battery. Since this elution of transition elements may become severe at a high voltage and a high temperature, it may bring about severe deterioration of a positive electrode at a high voltage and a high temperature. This deterioration may increase impedance (resistance) of a rechargeable lithium ion battery, deteriorating its cycle characteristic.

In order to solve this problem, a method of suppressing degradation of an electrolyte solution, a negative electrode, or a separator by interposing an aramid resin (wholly aromatic polyamide resin) layer between a positive electrode and the separator has been suggested. However, this method cannot suppress a metal from being eluted from a positive active material inside the positive electrode not contacting with the aramid resin layer.

SUMMARY

One aspect of this disclosure provides a positive electrode for a rechargeable lithium ion battery being capable of stably maintaining a positive active material even though the rechargeable lithium ion battery is charged and discharged at a high voltage and a high temperature but still having excellent cycle characteristics.

Another aspect of this disclosure provides a rechargeable lithium ion battery including the positive electrode.

According to one aspect of this disclosure, a positive electrode for a rechargeable lithium ion battery is provided that includes a positive electrode including a positive active material, a negative electrode including a negative active material. The positive electrode includes a lithium-manganese-based compound core and a heat-resistant polymer disposed on the lithium-manganese-based compound core, and the heat resistant polymer has a glass transition temperature (T_(g)) ranging from about 80 to about 400° C.

The heat resistant polymer with a glass transition temperature (T_(g)) ranging from about 80 to about 400° C. may be selected from the group consisting of a polyamide (PA) resin, a polyimide (PI) resin, a polyamideimide (PAI) resin, a polyacrylonitrile (PAN) resin, a polysulfone (PS) resin, a polybenzoimidazole (PBI) resin, a polytetrafluoroethylene (PTFE) resin, or a copolymer thereof, and a combination thereof. The polyamide resin may be a wholly aromatic polyamide-based resin. The heat resistant polymer may be included in an amount ranging from about 0.01 to 3 parts by weight based on 100 parts by weight of the lithium-manganese-based compound.

When the heat resistant polymer is disposed, a layer including the heat resistant polymer may have an average thickness ranging from about 0.002 to about 0.5 μm.

According to another aspect, a positive electrode for a rechargeable lithium ion battery may include a lithium-manganese-based compound core; and a heat resistant polymer and an inorganic metal compound disposed on the lithium-manganese-based compound core. The heat resistant polymer may have a glass transition temperature (T_(g)) ranging from about 80 to about 400° C.

The heat resistant polymer may be selected from the group consisting of a polyamide (PA) resin, a polyimide (PI) resin, a polyamideimide (PAI) resin, a polyacrylonitrile (PAN) resin, a polysulfone (PS) resin, a polybenzoimidazole (PBI) resin, a polytetrafluoroethylene (PTFE) resin, or a copolymer thereof, and a combination thereof. The polyamide resin may be a wholly aromatic polyamide-based resin. The heat resistant polymer may be included in an amount ranging from about 0.01 to about 3 parts by weight based on 100 parts by weight of the lithium-manganese-based compound.

The inorganic metal compound includes oxide, hydroxide, nitride, halide, sulfide, and a combination thereof. The inorganic metal compound includes a metal selected from the group consisting of Al, Ti, Zr, Mg, Si, Li, Zn, La, Nb, Ta, Ge, Y, Se, B, or a combination thereof. The inorganic metal compound includes Li_(2+2x)Zn_(1−x)GeO₄ (0<x<1), Li_(4−3x)Ga_(x)GeO₄ (0<x<1), La_(2/3−x)Li_(3x)TiO₃ (0.03≦x≦0.167), La_(1/3−x)Li_(3x)TaO₃ (0.025≦x≦0.167), La_(1/3−x)Li_(3x)NbO₃ (0≦x≦0.06), La_(1.3)Li_(1.7)Al_(0.3)(PO₄)₃, LiAlTa(PO₄)₃, LiAl_(0.4)Ge_(1.6)(PO₄)₃, Li_(1.4)Ti_(1.6)Y_(0.4)(PO₄)₃, Li₂O.SeO₂.B₂O₃, LiCl.Li_(1.4)M₂(PO₄)₃ (where M is Al, Ti, Ge, or mixtures thereof), Li_(x)PO_(y)N_(z) (x=2.9, y=3.3, and z=0.46), Li_(x)BO_(y)N_(z) (0<x<1, 0<y<1, and 0<z<1), Li₂S.P₂O₅, Li₂S.SiS₂, Li₂S.SiS₂.Li_(x)MO₄ (where M is Si, P, Ge, or mixtures thereof, and 0<x<1), Li₂S.SiS₂.Li₃PO₄, Li₂S.SiS₂.xMS_(y) (where M is Sn, Ta, Ti, or mixtures thereof, 0<x<1), Li₂S.SiS₂.Li₃N, Li₃N.SiS₂, or a combination thereof.

The heat resistant polymer is mixed with an inorganic metal compound in a ratio of about 5 to 50 parts by weight vs. about 95 to 50 parts by weight. When the heat resistant polymer is used with an inorganic metal compound, a layer including the heat resistant polymer and inorganic metal compound may have an average thickness ranging from about 0.002 to about 0.5 μm.

According to still another embodiment, a rechargeable lithium ion battery including the positive electrode is provided.

Accordingly, this disclosure can provide a positive electrode for a rechargeable lithium ion battery, which can stably maintain a lithium-manganese-based compound despite the charge and discharge at a high voltage and a high temperature and realizing excellent cycle characteristic due to its degradation suppression of the positive electrode.

In addition, since manganese is suppressed from being eluted, a separator and a negative electrode as well as a positive electrode may be suppressed from degradation.

DETAILED DESCRIPTION

Exemplary embodiments will hereinafter be described in detail. However, these embodiments are exemplary, and this disclosure is not limited thereto.

According to one embodiment, a positive electrode for a rechargeable lithium ion battery includes a lithium-manganese-based compound core and a heat resistant polymer disposed on the lithium-manganese-based compound core. The heat resistant polymer has a glass transition temperature (T_(g)) ranging from about 80 to about 400° C.

The lithium-manganese-based compound may have structural stability of an active material, which is determined by a reversible intercalation/deintercalation reaction of lithium ions. The lithium-manganese-based compound may be selected from the group consisting of compounds represented by the following Chemical Formulas 1 to 4, or a combination thereof.

Li_(x)Mn_(1−y)M_(y)A₂  [Chemical Formula 1]

Li_(x)Mn_(1−y)M_(y)O_(2−z)X_(z)  [Chemical Formula 2]

Li_(x)Mn₂O_(4−z)X_(z)  [Chemical Formula 3]

Li_(x)Mn_(2−y)M′_(y)A₄  [Chemical Formula 4]

In the above formula, 0.95≦x≦1.1, 0≦y≦0.7, 0≦z≦0.5, 0≦α≦2, M′ is at least one element selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and a rare earth element, A is at least one element selected from the group consisting of O, F, S, and P, and X is at least one element selected from the group consisting of F, S, and P.

When the lithium-manganese-based compound is used singularly, manganese may be eluted into an electrolyte solution as the charge and discharge proceed. The eluted manganese component may be deposited on the surface of a negative active material, for example, a carbon material, then receives electrons and promotes decomposition of the electrolyte solution through a reduction reaction on the negative active material, and increases resistance of a battery, bringing about degradation thereof. Accordingly, the manganese eluted on the surface of a negative active material sharply decreases reversible capacity in a lithium ion battery. This eluted manganese may become much more severe when stored at a high temperature.

However, when the heat resistant polymer is disposed on the lithium-manganese-based compound core according to one embodiment, manganese may be suppressed from elution against a lithium-manganese-based compound on the surface of a positive electrode as well as a lithium-manganese-based compound inside the positive electrode. Accordingly, an electrolyte solution may be slowly decomposed, suppressing manganese elution, degradation, gas generation, and the like, accomplishing excellent cycle characteristics of a lithium ion battery despite the charge and discharge at a high voltage and a high temperature.

The lithium-manganese-based compound may include the heat resistant polymer having a glass transition temperature ranging from about 80 to about 400° C. on the surface of a core particle. The heat resistant polymer having a glass transition temperature ranging from about 80 to about 400° C. may be at least one selected from the group consisting of, for example, a polyamide (PA) resin, a polyimide (PI) resin, a polyamideimide (PAI) resin, a polyacrylonitrile (PAN) resin, a polysulfone (PS) resin, a polybenzoimidazole (PBI) resin, a polytetrafluoroethylene (PTFE) resin, or a copolymer thereof. In particular, as for the glass transition temperature of the heat resistance polymer, the polyacrylonitrile (PAN) resin may have a glass transition temperature of about 87° C., the polyamideimide (PAI) resin may have a glass transition temperature of about 293° C., the polysulfone (PS) resin may have a glass transition temperature of about 185° C., and the polybenzoimidazole (PBI) resin may have a glass transition temperature of about 270° C.

The polyamide resin may be a wholly aromatic polyamide-based resin. The wholly aromatic polyamide-based resin (hereinafter referred to be a aramid resin) may without limitation include poly(phenyleneterephthalamide), poly(benzamide), poly(4,4′-benzanilidoterephthalamide), poly(phenylene-4,4′-biphenylene dicarbonic acid amide), poly(phenylene-2,6-naphthalenedicarbonic acid amide), poly(2-chloro-phenyleneterephthalamide), a phenyleneterephthalamide/2,6-chlorophenyleneterephthalamide multipolymer, and the like.

The aramid resin may have a melting point of about 180° C. or higher, and has excellent heat resistance. The aramid resin may have meta or para optical characteristics, and may be used singularly or as a combination of two or more.

When the heat resistant polymer is disposed on the surface of the lithium-manganese-based compound core, for example, a lithium-manganese-based compound, manganese may be suppressed from elution. Accordingly, a positive electrode as well as a separator and a negative electrode may be suppressed from degradation due to the eluted manganese. Accordingly, a rechargeable lithium ion battery may have improved cycle characteristics even at a high voltage and high temperature.

A method of disposing the heat resistant polymer having a glass transition temperature ranging from about 80 to about 400° C. on the surface of the lithium-manganese-based compound core may have no specific limit. For example, it may include a mechanical alloy method of mechanically grinding and combining the lithium-manganese-based compound core and the heat resistant polymer, a dry-method of mixing and heating the lithium-manganese-based compound core, and the heat resistance polymer or the inorganic metal compound, and a wet-method of dipping the lithium-manganese-based compound core in a coating liquid including the heat resistant polymer or the inorganic metal compound.

The heat resistant polymer may be used in an amount ranging from about 0.01 to about 3 parts by weight based on 100 parts by weight of the lithium-manganese-based compound. When it is used within the range, it may improve heat resistance and facilitate uniform coating of the inorganic metal compound on the surface of an active material.

The heat resistance polymerdisposed on the surface of the lithium-manganese-based compound core may be present in a layer having a thickness ranging from about 0.002 to about 0.5 μm. When the layer has a thickness within the range, the heat resistance polymer may be continuously coated on the core of the lithium-manganese-based compound, and thereby bring about a uniform heat resistance characteristics.

According to one embodiment, an inorganic metal compound as well as the heat resistance polymer having a glass transition temperature ranging from about 80 to about 400° C. may be coated on the surface of the lithium-manganese-based compound core particle.

The lithium-manganese-based compound and the heat resistant polymer having a glass transition temperature ranging from about 80 to about 400° C. may be the same as aforementioned, and is not illustrated again here.

The inorganic metal compound may include at least one compound selected from the group consisting of oxide, hydroxide, nitride, halide, and sulfide; or at least one metal element selected from the group consisting of Al, Ti, Zr, Mg, Si, Li, Zn, La, Nb, Ta, Ge, Y, Se, B, or a combination thereof. Since the inorganic metal compound has excellent heat resistance and improves the electrolyte solution impregnating property, it may simultaneously improve heat resistance and ion conductivity when it is used with the inorganic metal compound.

The inorganic metal compound may be selected from the group consisting of, for example, Li_(2+2x)Zn_(1−x)GeO₄ (0<x<1), Li_(4−3x)Ga_(x)GeO₄ (0<x<1), La_(2/3−x)Li_(3x)TiO₃ (0.03≦x≦0.167), La_(1/3x)Li_(3x)TaO₃ (0.025≦x≦0.167), La_(1/3x)Li_(3x)NbO₃ (0≦x≦0.06), La_(1.3)Li_(1.7)Al_(0.3) PO₄₃, LiAlTa PO₄₃, LiAl_(0.4)Ge_(1.6) PO₄₃, Li_(1.4)Ti_(1.6)Y_(0.4) PO₄₃, Li₂O.SeO₂.B₂O₃, LiCl.Li_(1.4)M₂ PO₄₃ (where M is Al, Ti, Ge, or a mixture thereof), Li_(X)PO_(y)N_(Z) (where x=2.9, y=3.3, and z=0.46), Li_(x)BO_(y)N_(Z) (where 0<x<1, 0<y<1, and 0<z<1), Li₂S.P₂O₅, Li₂S.SiS₂, Li₂S.SiS₂.Li_(x)MO₄ (where M is Si, P, Ge, or a mixture thereof, and 0<x<1), Li₂S.SiS₂.Li₃PO₄, Li₂S.SiS₂.xMS_(y) (where M is Sn, Ta, Ti, or a mixture thereof, and 0<x<1), Li₂S.SiS.Li₃N, Li₃N.SiS₂, or a combination thereof.

The heat resistant polymer and the inorganic metal compound are mixed together in a ratio of about 5 to 50 parts by weight vs. about 95 to 50 parts by weight. In another embodiment, the ratio may be in a range of about 10 to 30 parts by weight vs. about 90 to 70 parts by weight, but in still another embodiment, they are mixed in a ratio of about 15 to 25 parts by weight vs. about 85 to 75 parts by weight. In particular, when the inorganic metal compound is included within the range, it may prevent the area of a coating layer on the core of a lithium-manganese-based compound from becoming too dense, improving performance of a rechargeable lithium ion battery. In addition, it may improve ion conductivity and heat resistant effects as well as the impregnation effect of an electrolyte solution.

There is no particular limit to a method of disposing the heat resistance polymer and the inorganic metal compound on the core of the lithium-manganese-based compound. For example, it may include a mechanical alloy method of mechanically grinding and combining the lithium-manganese-based compound core and the heat resistant polymer or the inorganic metal compound, a dry-method of mixing and heating the lithium-manganese-based compound core and the heat resistant polymer or the inorganic metal compound, and a wet-method of dipping the lithium-manganese-based compound core in a coating liquid including the heat resistant polymer or the inorganic metal compound. In addition, after the positive active material particles are prepared into a positive electrode, the positive electrode is dipped into a coating liquid including a polyamide resin or an inorganic metal compound to coat the positive active material particles therein.

In addition, a layer including the heat resistant polymer and inorganic metal compound may have a thickness ranging from about 0.002 to about 0.5 μm on the core of the lithium-manganese-based compound. When it has a thickness within the range, it may have a continuous coating effect as aforementioned, bring about uniform heat-resistant characteristics.

When a heat resistant polymer having a glass transition temperature ranging from about 80 to about 400° C. is disposed with an inorganic metal compound on the core of a lithium-manganese-based compound, a layer on the core becomes extremely thick, deteriorating reaction of a positive electrode and an electrolyte solution. Accordingly, a layer on the core may have an average thickness ranging from about 0.002 to about 0.5 μm, and in another embodiment, from about 0.002 to about 0.2 μm. The thickness can be measured using TEM (transmission electron microscopy). In other words, a TEM photograph may make it possible to distinguish the core of the lithium manganese-based active material from a layer of the heat resistance polymer or the heat resistance polymer and inorganic metal compound. The minimum and maximum thicknesses of the layer are measured several times, for example, measured ten times, and calculated for an average.

In addition, a heat resistant polymer having a glass transition temperature ranging from about 80 to about 400° C. or an inorganic metal compound as well as the heat resistant polymer may be coated on a positive electrode including the positive active material.

According to one embodiment, a rechargeable lithium ion battery including the positive electrode is provided. Hereinafter, the rechargeable lithium ion battery according to one embodiment is illustrated.

According to the embodiment, a rechargeable lithium ion battery may be a coin type, a button type, a sheet type, a cylinder type, a flat type, a prismatic type, and the like. The rechargeable lithium ion battery may include a positive electrode, a negative electrode, an electrolyte, a separator, and the like.

The positive electrode for a rechargeable lithium ion battery may be the same as aforementioned, and may not be illustrated again.

Examples of the negative electrode may include a graphite-based carbon material, silicon, tin, a silicon alloy, a tin alloy, silicon oxide, lithium vanadium oxide, and the like as an active material, and in particular, a compound that can be capable of being alloyed with lithium, such as silicon, tin, a silicon alloy, and a tin alloy, or silicon oxide, lithium vanadium oxide, and the like as an active material.

The graphite-based carbon material has capacity density ranging from about 560 to about 630 mAh/cm³, while silicon, tin, a silicon alloy, a tin alloy, silicon oxide, lithium vanadium oxide, and the like have capacity density of about 850 mAh/cm³ or more. These materials may bring about down-sizing and high capacity of a battery. On the other hand, these negative active materials may be used singularly or as a combination of two or more.

The positive and negative electrodes may be prepared by adding an additive that is appropriately selected from the group consisting of an electrically conductive material, a binder, a filler, a dispersing agent, and an ionic electrically conductive material to the aforementioned active material powder.

Examples of the electrically conductive material may include graphite, carbon black, acetylene black, ketjen black, carbon fiber, metal powder, and the like. Examples of the binder may include polytetrafluoroethylene, polyvinylidene fluoride (PVdF), polyethylene, and the like.

The positive or negative electrode may be fabricated, for example, by preparing slurry or paste through addition of a mixture of an electrically active material with various additives to a solvent such as water, an organic solvent, and the like, coating the slurry or paste on an electrode-supporting substrate by a doctor blade method and the like, and compressing it with a compressing roller and the like.

The electrode-supporting substrate may include a foil, a sheet, or a net made of copper, nickel, stainless steel, and the like, or a sheet or a net made of carbon fiber. On the other hand, the negative electrode may be fabricated using no electrode-supporting substrate but by pressing/compressing it into a pellet.

Examples of the electrolyte may include a non-aqueous electrolyte prepared by dissolving a lithium salt in an organic solvent, a polymer electrolyte, an inorganic solid electrolyte, a composite material of a polymer electrolyte and an inorganic solid electrolyte, and the like.

The non-aqueous electrolyte may include a cyclic carbonate such as ethylene carbonate, propylene carbonate, vinylene carbonate, and the like, a linear carbonate such as dimethyl carbonate, diethylcarbonate, methylethylcarbonate, and the like, γ-lactones such asγ-butyl lactone and the like, linear ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, and the like, cyclic ethers such as tetrahydrofurans, and nitriles such as acetonitrile and the like as a solvent. These solvents may be used singularly or as a combination of two or more.

The non-aqueous electrolyte may include a lithium salt as a solute, for example, LiAsF₆, LiBF₄, LiPF₆, LiAlCl₄, LiClO₄, LiCF₃SO₃, LiSbF₆, LiSCN, LiCl, LiC₆H₅SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, LiC₄P₉SO₃, and the like.

Examples of the separator may include a porous film made of a polyolefin such as polypropylene, polyethylene, or the like. In addition, a polyamide layer may be disposed on the surface of the porous film. This polyamide layer may include an inorganic metal compound.

Hereinafter, examples and comparative examples of this disclosure are illustrated. These following examples are illustrated in more detail as exemplary embodiments, but this disclosure may not be limited thereto.

Example 1

0.25 wt % of a polyimide (PI) resin was dissolved in N-methyl-2-pyrrolidone (NMP), and about 94.75 wt % of LiMn₂O₄ having a particle size of about 10 μm was added thereto. The resulting mixture was agitated with an agitator. Next, it was filtrated to separate a solid from the solution. The solid was vacuum-dried at 160° C. The LiMn₂O₄ included a 0.005 μm-thick layer on the core. The solid was mixed with acetylene black and polyvinylidene fluoride (PVdF) in a ratio of 94:3:3 to prepare a positive active material slurry. The slurry was coated on an aluminum substrate, fabricating a positive electrode.

On the other hand, about 3 wt % of graphite powder and 3 wt % of polyvinylidene fluoride (PVdF) as a binder were added to N-methyl-2-pyrrolidone to prepare a negative active material slurry. The negative active material slurry was coated on a copper foil as a negative electrode current collector, fabricating a negative electrode.

Then, polypropylene as a separator was disposed between the positive and negative electrodes, and a non-aqueous electrolyte was injected therein, fabricating a coin-type rechargeable lithium ion battery cell. The non-aqueous electrolyte was prepared by dissolving LiPF₆ in a concentration of 1.50 mol/L in a solvent of ethylenecarbonate and diethylcarbonate mixed in a ratio of 3:7.

Example 2

PI, a heat resistant polymer, and LiAlTa(PO₄)₃, an inorganic metal compound, were mixed in a weight ratio 1:3. The mixture was coated on the surface of a LiMn₂O₄ particle. Next, a positive electrode was fabricated according to the same method as in Example 1. The positive electrode was used to fabricate a rechargeable lithium ion battery cell according to the same method as in Example 1.

Example 3

A rechargeable lithium ion battery cell was fabricated according to the same method as in Example 1 by including a positive electrode prepared by coating PI, a heat resistant polymer, on the surface of a LiNi_(0.5)CO_(0.2)Mn_(0.3)O₂ particle.

Example 4

A rechargeable lithium ion battery cell was fabricated according to the same method as in Example 1 by including a positive electrode prepared by mixing PI, a heat resistant polymer, and LiAlTa(PO₄)₃, an inorganic metal compound, in a weight ratio of 1:3 and coating the mixture on the surface of a LiNi_(0.5)CO_(0.2)Mn_(0.3)O₂ particle.

Comparative Example 1

A rechargeable lithium ion battery cell was fabricated by including a positive electrode prepared by mixing LiMn₂O₄, acetylene black as a carbon-based conductive material, and a PVdF binder in a ratio of 94:3:3 into a slurry and coating the slurry on an aluminum substrate according to the same method as in Example 1.

Comparative Example 2

A rechargeable lithium ion battery cell was fabricated by including a positive electrode prepared by mixing LiNi_(0.5)CO_(0.2)Mn_(0.3)O₂, acetylene black as a carbon-based conductive material, and a PVdF binder in a ratio of 94:3:3 into a slurry and coating the slurry on an aluminum substrate according to the same method as in Example 1.

Battery Cell Performance Evaluation

The coin-type rechargeable lithium ion battery cells according to Examples 1 to 4 and Comparative Examples 1 and 2 were charged and discharged. The charge and discharge experiment was performed once with a current density of 0.1 C, a charging cut-off voltage of 4.3V (Li/Li⁺), and a discharging cut-off voltage of 3.0V (Li/Li⁺).

Next, the coin-type rechargeable lithium ion battery cells were respectively charged and discharged once at a charge and discharge current density of 0.2 C and 0.5 C under the aforementioned charge and discharge cut-off voltage conditions, and then 50 times at a current density of 1.0 C. These charge and discharge experiments were all performed in a 60° C. high temperature chamber.

All the charges and discharges were performed with a constant current. After the 50 cycle experiment, the first cycle discharge capacity (initial capacity) and charge and discharge efficiency (discharge capacity/charge capacity, initial efficiency) were calculated. In addition, the capacity retention was calculated through a capacity ratio (50th/1st) by dividing the 50th cycle discharge capacity by 0.1 C of the first cycle discharge capacity. Each binder composition was evaluated by fabricating more than three coin-type rechargeable lithium ion battery cells and calculating the average value. The results are provided in the following Table 1.

TABLE 1 Initial Initial discharge 50^(th) cycle capacity capacity capacity (mAh/g) efficiency (%) retention (%) Example 1 115.9 80.6 82.6 Example 2 117.2 80.5 88.1 Comparative 121.9 71.8 54.5 Example 1 Example 3 158.2 88.6 89.4 Example 4 156.2 88.2 92.1 Comparative 160.4 87.5 80.9 Example 2

As shown in Table 1, rechargeable lithium ion battery cells fabricated by coating a lithium manganese-based oxide particle with an inorganic metal compound as well as a heat resistant polymer having a glass transition temperature (T_(g)) ranging from 80 to 400° C. or the heat resistant polymer according to Examples 1 to 4 had excellent performance compared with the ones according to Comparative Examples 1 and 2.

Specifically, compared with the rechargeable lithium ion battery cells including LiMn₂O₄ as a lithium manganese-based compound core according to Examples 1 and 2 and Comparative Example 1, the one including a heat resistant polymer on the surface of a LiMn₂O₄ particle according to Example 1 and the one including a heat resistant polymer and an inorganic metal compound on the surface of a LiMn₂O₄ particle according to Example 2 had about 30% higher capacity retention than the one including only LiMn₂O₄ according to Comparative Example 1.

In addition, compared with Examples 3 and 4 and Comparative Example 2 including LiNi_(0.5)CO_(0.2)Mn_(0.3)O₂ as a lithium manganese-based compound core, rechargeable lithium ion battery cells including a heat resistant polymer coated on the surface of a LiNi_(0.5)CO_(0.2)Mn_(0.3)O₂ particle and Example 4 including a heat resistant polymer and an inorganic metal compound coated on the surface of a LiNi_(0.5)CO_(0.2)Mn_(0.3)O₂ particle had about 10% higher capacity retention than Comparative Example 2 including only LiNi_(0.5)CO_(0.2)Mn_(0.3)O₂.

Therefore, when a rechargeable lithium battery was fabricated by using a positive active material including a heat resistant polymer or both of a heat resistant polymer and an inorganic metal compound on the core of the lithium manganese-base compound, the lithium manganese-base compound had an effect of suppressing manganese elution, and thereby improved battery performance from the examples and comparative examples.

While this disclosure has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A positive electrode for a rechargeable lithium ion battery, comprising: a lithium-manganese-based compound core; and a heat resistant polymer disposed on the lithium-manganese-based compound core, wherein the heat resistance polymer has a glass transition temperature (T_(g)) ranging from about 80 to about 400° C.
 2. The positive electrode of claim 1, wherein the heat resistant polymer is selected from the group consisting of a polyamide (PA) resin, a polyimide (PI) resin, a polyamideimide (PAI) resin, a polyacrylonitrile (PAN) resin, a polysulfone (PS) resin, a polybenzoimidazole (PBI) resin, a polytetrafluoroethylene (PTFE) resin, or a copolymer thereof, and a combination thereof.
 3. The positive electrode of claim 1, wherein the polyamide resin is a wholly aromatic polyamide-based resin.
 4. The positive electrode of claim 1, wherein the heat resistant polymer is comprised in a range of about 0.01 to about 3 parts by weight based on 100 parts by weight of the lithium-manganese-based compound.
 5. The positive electrode of claim 1, wherein a layer comprising the heat resistant polymer has an average thickness ranging from about 0.002 to about 0.5 μm.
 6. A positive electrode for a rechargeable lithium ion battery, comprising: a lithium-manganese-based compound core; and a heat resistant polymer and an inorganic metal compound on the surface of the manganese-based compound core, wherein the heat resistant polymer has a glass transition temperature ranging from about 80 to about 400° C.
 7. The positive electrode of claim 6, wherein the heat resistant polymer is selected from the group consisting of a polyamide (PA) resin, a polyimide (PI) resin, a polyamideimide (PAI) resin, a polyacrylonitrile (PAN) resin, a polysulfone (PS) resin, a polybenzoimidazole (PBI) resin, a polytetrafluoroethylene (PTFE) resin, or a copolymer thereof, and a combination thereof.
 8. The positive electrode of claim 6, wherein the polyamide resin is a wholly aromatic polyamide-based resin.
 9. The positive electrode of claim 6, wherein the heat resistant polymer is comprised in a range of about 0.01 to about 3 parts by weight based on 100 parts by weight of the lithium-manganese-based compound.
 10. The positive electrode of claim 6, wherein the inorganic metal compound is selected from the group consisting of oxide, hydroxide, nitride, halide, sulfide, and a combination thereof.
 11. The positive electrode of claim 6, wherein the inorganic metal compound comprises a metal selected from the group consisting of Al, Ti, Zr, Mg, Si, Li, Zn, La, Nb, Ta, Ge, Y, Se, B, or a combination thereof.
 12. The positive electrode of claim 6, wherein the inorganic metal compound is selected from the group consisting of Li_(2+2x)Zn_(1−x)GeO₄ (0<x<1), Li_(4−3x)Ga_(x)GeO₄ (0<x<1), La_(2/3−x)Li_(3x)TiO₃ (0.03≦x≦0.167), La_(1/3−x)Li_(3x)TaO₃ (0.025≦x≦0.167), La_(1/3−x)Li_(3x)NbO₃ (0≦x≦0.06), La_(1.3)Li_(1.7)Al_(0.3) PO₄₃, LiAlTa PO₄₃, LiAl_(0.4)Ge_(1.6) PO₄₃, Li_(1.4)Ti_(1.6)Y_(0.4) PO₄₃, Li₂O.SeO₂.B₂O₃, LiCl.Li_(1.4)M₂ PO₄₃ (M is Al, Ti, Ge, or a mixture thereof), Li_(x)PO_(y)N_(z) (x=2.9, y=3.3, and z=0.46), Li_(x)BO_(y)N_(z) (0<x<1, 0<y<1, and 0<z<1), Li₂S.P₂O₅, Li₂S.SiS₂, Li₂S.SiS₂.Li_(x)MO₄ (M is Si, P, Ge, or a mixture thereof, and 0<x<1), Li₂S.SiS₂.Li₃PO₄, Li₂S.SiS₂.xMS_(y) (M is Sn, Ta, Ti, or a mixture thereof, and 0<x<1), Li₂S.SiS.Li₃N, Li₃N.SiS₂, or a combination thereof.
 13. The positive electrode of claim 6, wherein the heat resistant polymer and the inorganic metal compound are mixed in a ratio ranging from about 5 to 50 parts by weight vs. about 95 to 50 parts by weight.
 14. The positive electrode of claim 6 wherein a layer comprising the heat resistant polymer and the inorganic metal compound has an average thickness ranging from about 0.002 to about 0.5 μm.
 15. A rechargeable lithium ion battery comprising the positive electrode of claim
 1. 16. A rechargeable lithium ion battery comprising the positive electrode of claim
 6. 